From time to time during the production of ethylene homopolymers and copolymers in a commercial reactor, it is necessary to shut down the polymerization reactor for periodic maintenance and cleaning. In order to re-initiate polymerization following such shutdowns when supported chromium oxide is intended to be used as catalyst in the process, it is frequently necessary to introduce a quantity of such catalyst into the reactor that is many times that required to carry on reaction at a normal rate due to the presence of moisture and other catalyst poisons in the reactor which react with the catalyst and inhibit polymerization. In spite of the large amount of catalyst employed, polymerization usually does not initiate for at least 4 to 12 hours following introduction of the catalyst into the reactor. Further, once polymerization does commence, it often proceeds very rapidly due to the large amount of catalyst present. Indeed, so rapid is the polymerization (and heat generation) that it presents the possibility of a runaway reaction. In the case of fluidized bed polymerizations, this results in fouling of the reactor due to fusing of the rapidly-formed polymer and/or sheeting of the polymer on the reactor wall where mixing and heat removal are not as effective as in the center of the bed.
One procedure suggested to initiate the polymerization of ethylene smoothly and reliably and reduce the possibility of a runaway reaction is to commence polymerization with a supported silyl chromate catalyst and then switch to the supported chromium oxide catalyst after polymerization has begun. However, this procedure suffers from the disadvantage that the properties of the initial polymerization product differ from the desired polymer properties, and hence a great deal of waste product must be produced before the desired product can be obtained. As a result, polymerization must be continued for a time sufficient to provide at least 4 to 6 reactor turnovers before the desired product can be obtained (i.e., at least the equivalent of 4 to 6 times the volume of polymer in the reactor must be removed from the reactor following the initiation of polymerization with the chromium oxide catalyst before polymers having the desired properties can be obtained).